1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a plurality of transmission trains to be established by a selecting operation of a plurality of hydraulic engaging elements. In this specification, the term "vehicular transmission" means a transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
As this kind of control apparatus, there has hitherto been known the following one. Namely, the hydraulic pressure of a hydraulic engaging element on disengaging side to be disengaged at the time of speed changing (disengaging pressure) and the hydraulic pressure of a hydraulic engaging element on engaging side to be engaged at the time of speed changing (engaging pressure) are electronically controlled by using solenoid proportional valves. An input and output speed ratio (rotational speed of output shaft/rotational speed of input shaft) of a transmission is detected. The state of development of speed changing is discriminated by the input and output speed ratio. The disengaging pressure and the engaging pressure are thereby controlled.
In this kind of apparatus, there is also known an apparatus in which, at the time of downshifting, the disengaging pressure is first lowered to cause the engine to race by the slipping of the hydraulic engaging element on the disengaging side. When the input and output speed ratio has fallen below a predetermined value by the lowering thereof due to the racing of the engine, the engaging pressure is boosted or increased.
In order to perform the downshifting smoothly, it is necessary to engage the hydraulic engaging element on the engaging side when the input and output speed ratio of the transmission has lowered into a predetermined synchronous region which is set based on the gear ratio of the speed stage which is established by the engagement of the hydraulic engaging element on the engaging side. However, it takes time, though small in amount, for the hydraulic pressure of the hydraulic engaging element on the engaging side to actually rise since a boosting command of the engaging pressure has been issued. To allow for this response delay, it has conventionally been arranged to set the above-described predetermined value to a value higher than the synchronous region to thereby boost the engaging pressure before the input and output speed ratio lowers into the synchronous region.
The above-described response delay becomes short in a high vehicle speed and long in a low vehicle speed due to the influence of the centrifugal force. However, the change in response delay with the vehicle speed is not conventionally taken into consideration. As a result, the hydraulic engaging element on the engaging side is sometimes engaged behind the synchronous point of time at a high vehicle speed or engaged before the synchronous point of time at a low vehicle speed. Shocks are thus likely to occur.
Further, when the engine temperature is low, a fast idle operation is performed and, therefore, the engine output becomes large. If the engine output is large, the rate of increase in the rotational speed of the input shaft becomes large when slipping occurs to the hydraulic engaging element on the disengaging side, resulting in a rapid decrease in the input and output speed ratio. As a consequence, when the engine temperature is low, the decrease in the input and output speed ratio becomes rapid. In the conventional system, however, no consideration is taken of the change in the decrease speed in the input and output speed ratio due to the engine temperature. Therefore, at the time of low temperature, the hydraulic engaging element on the engaging side is engaged behind the synchronous point of time, and shocks are likely to occur.
In view of the above point, the present invention has a first object of providing a control apparatus in which the hydraulic engaging element on the engaging side is engaged at the synchronous point of time so that smooth downshifting can be performed.
Further, as a control apparatus for a hydraulically operated vehicular transmission, there is conventionally known one having means for controlling stepwise speed changing for downshifting one stage at a time such that during downshifting control to one speed stage which is a stage lower, upon receipt of a downshifting command to another speed stage which is a stage lower than said one speed stage, a start of downshifting control to said another speed stage is delayed until the downshifting control to said one speed stage is completed.
In this kind of control system for stepwise speed changing, downshifting to a two-stage lower speed stage which is required by the driver of the vehicle, e.g., from the fourth speed stage to the second speed stage, is performed in two downshifting operations of fourth speed.fwdarw.to third speed and third speed.fwdarw.second speed. It has therefore an advantage in that the shocks can be reduced and the control system can be simplified.
On the other hand, since the downshifting is performed one step at a time, a longer time is required than the one in which downshifting is made directly to the two-stage lower speed stage. This conventional system sometimes causes the driver to feel that the time lag for switching the driving force is large.
In view of the above point, the present invention has a second object of shortening the time of downshifting in a control system for stepwise speed changing.